[css-color-4] Avoid infinite loop in gamut reduction with whiter-then-white or blacker-than-black, fix #6999

Author: svgeesus

css-color-4/Overview.bs

@@ -5253,6 +5253,10 @@ Gamut Mapping
 
 		<ol>
 			<li>let |origin_OKLCH| be |origin| converted to the OKLCH color space</li>
+			<li>if the Lightness of |origin_OKLCH| is greater than or equal to 100%,
+				return { 1 1 1 origin.alpha } in |destination|</li>
+			<li>if the Lightness of |origin_OKLCH| is less than than or equal to 0%,
+					return { 0 0 0 origin.alpha } in |destination|</li>
 			<li>let inGamut(|color|) be a function which returns true if, when passed a color,
 				that color is inside the gamut of |destination|
 			</li>
@@ -5275,7 +5279,7 @@ Gamut Mapping
 						<ol>
 							<li>set |clipped| to clip(|current|)</li>
 							<li>set |E| to delta(|clipped|, |current|)</li>
-							<li>if |E| < |JND| convert |clipped| to |destination| and return it as the gamut mapped color</li>
+							<li>if |E| < |JND| return |clipped| as the gamut mapped color</li>
 							<li>otherwise, set |max| to |chroma| and continue to repeat these steps</li>
 						</ol>
 					</li>